The technology disclosed here relates to a motor vehicle with a cryogenic pressure vessel and to a method for refueling a cryogenic pressure vessel of a motor vehicle.
Cryogenic pressure vessels are known from the prior art. Such a pressure vessel has an inner vessel and an outer vessel which surrounds the latter with the formation of a super-insulated (for example evacuated) (intermediate) space. Cryogenic pressure vessels are used, for example, for motor vehicles in which a fuel which is gaseous under ambient environmental conditions is stored cryogenically and therefore in the liquid or supercritical state of aggregation, thus substantially at a significantly higher density in comparison to the environmental conditions. Such fuels, for example hydrogen or compressed natural gas, are stored in the cryogenic pressure vessels at, for example, temperatures of approx. 30 K to 360 K. The pressure vessels therefore require extremely good thermal insulation in order to prevent as far as possible the undesirable introduction of heat into the cryogenically stored medium. Highly effective insulation casings (for example vacuum casings) are therefore provided. For example, EP 1 546 601 B1 discloses such a pressure vessel. Furthermore, DE 10 2012 218 989 A1 and DE 11 2010 004 462 T5 are prior art.
If the thermal insulation of the pressure vessel is inadequate or if the thermal insulation is damaged, the stored fuel heats up slowly. At the same time, the pressure in the pressure vessel rises slowly. If a limit pressure is exceeded, the fuel has to escape via suitable safety devices in order to avoid bursting the cryogenic pressure vessel. For this purpose, use is made of, for example, what is referred to as a blow-off management system or boil-off management system (referred to as BMS below). These systems permit fuel to escape, wherein the released fuel is converted, for example, in a catalytic converter. Furthermore, use is additionally made of mechanical safety valves (SVT) or pressure control valves and bursting disks which, mounted downstream of the BMS, can discharge the fuel.
If a pressure vessel having the maximum storage density of an undamaged pressure vessel despite having defective thermal insulation is refueled, the abovementioned safety devices gradually become active. If the released fuel cannot be converted by the BMS, the fuel is released unused into the environment. An explosive or at least combustible mixture could then arise. Therefore, the use of the vehicle with defective thermal insulation should cease and the pressure vessel should be immediately replaced.
It is an object of the technology disclosed here to improve a cryogenic pressure vessel or to provide an alternative configuration. The object is achieved by a motor vehicle with a cryogenic pressure vessel in accordance with embodiments of the invention.
The technology disclosed here relates to a motor vehicle. The motor vehicle includes one or more cryogenic pressure vessels. The cryogenic pressure vessel is, for example, a cryogenic pressure vessel that has been described in the introductory part. In particular, it is suitable for storing fuel, preferably hydrogen, in the supercritical range, i.e. preferably in the design or operating temperature window of approx. 30 K to approx. 360 K, particularly preferably in the temperature window of approx. 40 K to approx. 330 K. The cryogenic pressure vessel preferably stores the fuel at the same time within a pressure range of approx. 5 bar to approx. 1000 bar, preferably within a pressure range of approx. 5 bar to approx. 700 bar, and particularly preferably of approx. 20 bar to approx. 350 bar. A cryogenic pressure vessel is suitable in particular for storing the fuel at temperatures which lie significantly below the operating temperature of the motor vehicle (i.e. the temperature range of the vehicle surroundings in which the vehicle is intended to be operated), for example at least 50 Kelvin (K), preferably at least 100 Kelvin (K) or at least 150 Kelvin (K) below the operating temperature of the motor vehicle (as a rule approx. −40° C. to approx. +85° C.).
The cryogenic pressure vessel includes, inter alia, an inner vessel storing a fluid and an outer vessel which surrounds the inner vessel. The inner vessel is held in the outer vessel in a manner as thermally insulated as possible. Thermal insulation V is arranged at least in regions between the inner vessel and the outer vessel. In addition to ideal or perfect insulation, the term “thermal insulation” also includes here thermal insulation at which a small heat exchange still takes place. The heat exchange can be of any type, for example heat conduction, heat radiation, heat convection, etc. The thermal insulation V can be designed, for example, as an evacuated space V.
Furthermore, at least one sensor for monitoring the thermal insulation V can be provided on the pressure vessel. The at least one sensor can be a pressure sensor which monitors the pressure in the evacuated space V. The sensor can also be a temperature sensor which monitors the temperature of the inner vessel, in the thermal insulation V, or of the outer vessel and, together with further parameters, such as, for example, the fluid density in the inner vessel and/or the inner vessel pressure, permits conclusions to be drawn regarding the thermal insulation V. However, other suitable monitoring sensors or monitoring devices can also be provided.
The motor vehicle furthermore includes one or more controllers. At least one controller is designed to interrupt refueling of the motor vehicle if, in the event of damaged thermal insulation V, a lower fluid density limit value DUB for the fluid in the inner vessel is exceeded.
The fluid density D in the inner vessel is the quotient of mass of fluid in the inner vessel divided by the inner vessel volume. The lower fluid density limit value DUB of the fluid in the inner vessel is a limit value which should be taken into consideration during the refueling of a pressure vessel with damaged thermal insulation. The lower fluid density limit value DUB indicates up to which fluid density the inner vessel may be maximally (and in particular cryogenically) refueled because of the damaged thermal insulation.
The lower fluid density limit value DUB can be selected in such a manner that, even when the vehicle/the pressure tank is operated (or else is stopped) at temperatures at the upper edge of the temperature window, for example at environmental temperatures, a vessel inner pressure which lies above the maximum operating pressure does not arise in the inner vessel after the refueling because of the thermal expansion of the fluid.
In an advantageous configuration, the lower fluid density limit value DUB varies with the amount of damage of the thermal insulation V. If, for example, there is little damage to the thermal insulation V, the controller interrupts the refueling of the motor vehicle at a higher lower fluid density limit value DUB than in the case in which there is greater damage to the thermal insulation V.
The lower fluid density limit value DUB is lower than an upper fluid density limit value DOB. The upper fluid density limit value DOB is the limit value for the fluid in the inner vessel in the case of cryogenic refueling of the inner vessel having intact thermal insulation V. The upper fluid density limit value DOB can be, for example, the fluid density which arises if the inner pressure vessel is cryogenically refueled at a fluid temperature in the lower range of the temperature window or else below the latter (for example approx. 30 K to approx. 50 K) until the maximum operating pressure of the inner pressure vessel arises in the inner pressure vessel.
Intact thermal insulation V has thermal insulation which is original or functional for the normal operation. The intact thermal insulation V is designed here in such a manner that heat gradually penetrating the inner vessel allows the temperature and the internal pressure in the inner vessel to slowly rise. On account of the slow rise, the BMS has sufficient time to convert the fuel. Since the BMS can convert the fuel completely, the mechanical safety valves (SVT) and bursting disks do not release any fuel into the environment during this normal operation. The quantities of hydrogen which are converted via the BMS are comparatively low here. Even the internal pressure in the vessel does not rise above the maximum operating pressure of the inner vessel. In association therewith, the BMS is designed in such a manner that it can always satisfactorily convert fuel during the normal operation in order to avoid a rise in pressure, which is imminent due to penetrating heat, such that the maximum operating pressure is not exceeded. For this purpose, the BMS is provided with a certain margin of safety. In other words, the BMS can convert more fuel than would actually occur during normal operation with intact thermal insulation V. Even small degradations can therefore be absorbed.
However, defective or damaged thermal insulation V is not capable of providing its functional thermal insulation which is required for normal operation of the cryogenic pressure vessel without release of fuel through the safety valve(s) in the design temperature window and design pressure range. The thermal insulation of damaged thermal insulation V is greatly damaged or greatly degraded to the extent that the BMS is no longer capable of sufficiently converting fuel during normal operation. In order to avoid a rise in pressure, which is imminent due to penetrating heat, above the maximum operating pressure, fuel has to be released by the safety valve(s).
For example, damaged thermal insulation V is present if the degraded thermal insulation property is less than approx. 50%, furthermore preferably less than approx. 75% and particularly preferably less than 90% of the thermal insulation property of the intact thermal insulation.
At least one controller can be connected to the at least one sensor. Furthermore, at least one controller can be designed to determine the state of the thermal insulation V from the signal of the at least one sensor or of the monitoring device(s). The term controller here is used in its conventional sense and comprises means for controlling and/or regulating the components disclosed here, wherein for simplification only the term controller is used here.
The motor vehicle can include a refueling valve which is designed to interrupt the inflow of fluid into the inner vessel during the refueling, for example if the lower fluid density limit value DUB for the fluid in the inner vessel is exceeded. Alternatively or additionally, the motor vehicle can include a communication interface which is suitable for transmitting a refueling termination signal and/or a refueling limiting signal to the refueling device. For example, the motor vehicle can directly and/or indirectly measure the fluid density in the inner vessel. With a constant volume of the inner tank, the fluid density can be determined by a combined measurement of pressure and temperature in the inner tank. If it is established that the lower fluid density limit value DUB for the fluid in the inner vessel is exceeded, a controller triggers an action of the refueling valve, as a result of which the valve closes and the inflow of fluid into the inner vessel is interrupted or limited. Alternatively or additionally, a controller can send an interruption signal to the refueling device which then, for its part, closes a valve.
The technology disclosed here also includes a method for refueling a cryogenic pressure vessel of a motor vehicle. The method comprises the following steps:                determining damage to thermal insulation V which is arranged at least in regions between an inner vessel and an outer vessel of the cryogenic pressure vessel; and        interrupting the refueling of the motor vehicle if, in the event of damaged thermal insulation V, a lower fluid density limit value DUB for the fluid in the inner vessel is exceeded, wherein the lower fluid density limit value DUB is lower than an upper fluid density limit value DOB for the fluid in the inner vessel in the case of refueling of the inner vessel with intact thermal insulation V.        
The method can be distinguished in that the lower fluid density limit value DUB is selected in such a manner that the inner vessel, even in the uninsulated state, can store the fluid filled cryogenically in the vessel without the maximally permissible inner vessel pressure Pmax being exceeded. In other words, the fluid density limit value DUB is selected in such a manner that the inner vessel can store the cryogenically filled quantity of fluid over the entire temperature window without the maximally permissible inner vessel pressure Pmax being exceeded.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.